In most cases you'd have to pay the platform owners many millions of dollars for the rights to create an emulator for those platforms, then you'd need to pay many more millions for the right to each individual game which is just not going to happen. At best you might get first party releases for the platforms by the platform owners, I think Sega did this with a lot of their Mega Drive games, but anything more than that is going to be too much of a legal minefield for anyone to pursue.

I know you say 'host' so are you talking about someone going through the rigmarole of sorting out the rights to all those ancient games so that they're available on Steam? Or would you also be suggesting simply putting emulators on Steam and encouraging piracy (under the guise of pretending everyone is getting hold of copies of ancient games legally)?

I am sure owners of the rights to the software and indeed the relevant hardware would love a platform like steam to promote thier work.

emulators already exist, i'm sure a large operation like Steam could organise the licensing etc and then some of these classics, which are currently being played the world over by means of piracy would finally have a legal and clean market place where they could reach a whole new generation of games lovers.

There is potential to make money for steam, for the emulator makers, money for the games publishers/developers and for the owners of the original platforms.

As much as they would "love" to have Steam (though you really mean Valve, Steam is just the service) handle this process, they'd love even more the deep pockets that Valve has. The financial issues alone would prevent this from ever happening, without even getting started on the potential for legal issues that Valve would be opening the door for if they started hosting emulators that could be used to play games they don't have the rights to.

I am sure owners of the rights to the software and indeed the relevant hardware would love a platform like steam to promote thier work.

You'd be wrong, there are a lot of costly issues supporting old games, there's also other costs like legal issues, marketing, development of the emulator and compatibility testing, and it would be difficult to cover all the costs with sales from old games that probably wont be bought at more than a few dollars and that few people would be interested in. That's not the worst of it either they need to track down all the game data which can often be very difficult as a lot of the developers that had it would have gone out of business many years ago and liquidators rarely know what to do with digital assets, especially 10+ years ago, it's a lot of effort to re-release an old game and it's unlikely to pay off.

emulators already exist, i'm sure a large operation like Steam could organise the licensing etc and then some of these classics, which are currently being played the world over by means of piracy would finally have a legal and clean market place where they could reach a whole new generation of games lovers.

Except that the emulators are rarely legal to begin with or at best occupy some legal grey area, they are often made without the permission of the platform owners and represent a huge legal risk for Valve to get involved with, any actual emulators that are likely to be sold are going to need to be official products of the platform owners, and the platform owners aren't going to want to be spending millions of dollars developing them.

There is potential to make money for steam, for the emulator makers, money for the games publishers/developers and for the owners of the original platforms.

Valve would stand to make money from a legal product sure, but the emulator makes stand to lose millions that they probably don't have in legal disputes with the platform owners, the game publishers/developers are probably going to lose money too since the platform owners will likely charge a licensing fee for use of the emulator, and while a lot of money looks like it would go to the platform owners they too stand to lose a lot of money due to the costs of writing an emulator and supporting games that use it compared to the relatively low price they would be able to sell the games for and the low amount of demand for such games.

Not really GOG.com would be better suited, and I'm not sure if Steam's method of content delivery and execution would work well with all the low level requirements of emulators such as driver manipulation and hardware specific performance hacks.

If you ever find yourself looking at a situation that seems like a win/win/win, and it hasn't already happened when you believe it easily could, it's probably a better indication that there's some part of the situation you don't understand or are unaware of rather than it simply not occurring to anyone else before.